1. Field of the Invention
The present invention relates to cladded optical fibers for transmission of electromagnetic energy in the infrared region having a protectice coating and to a method for manufacture of such fibers.
2. Description of the Prior Art
There has been great interest in the past few years in optical fibers capable of conducting infrared (IR) energy. The most promising materials are polycrystalline infrared fibers which have been found to be useful as optical waveguides. The most common materials investigated in the prior art are thallium bromo-iodide (TlBrI, known as KRS-5), thallium bromide (TlBr), silver chloride (AgCl), potassium bromide (KBr), and potassium chloride (KCl).
The most common application of waveguides formed from these materials is in conjunction with CO.sub.2 lasers with the fibers used to deliver the 10.6 .mu.m laser energy to remote or otherwise inaccessible locations for heating or for machining. The IR waveguides are also useful for transmitting infrared energy in pyrometry or spectroscopy from a source of heat to remote heat sensitive detectors. There is also considerable interest in using such fibers for long distance communications as an alternative to the more commonly used silica fibers.
Perhaps the most promising and useful application of polycrystalline IR fiber waveguides is in surgical procedures. Through the use of very small diameter fibers, from about 75 micrometers (.mu.m) to 1 or 2 millimeters (mm), laser energy may be transmitted into arteries, veins, joints, the eye, and other parts of the body. All living tissues strongly absorb the 10.6 .mu.m energy from the carbon dioxide laser due to the high absorption of the water contained in the tissue and therefore there is great potential for cutting with limited hemostasis and for tissue ablation. Current practical techniques of CO.sub.2 laser energy delivery are limited to direct line of sight or by reflections from mirrors or prisms.
Most infrared fiber waveguides have been produced from a billet or preform of the bulk crystal material. The preform is heated and extruded through a suitable die to obtain the fibers. This process results in a polycrystalline form having a grain size which is a function of the temperature, rate of extrusion, and other factors. Both the physical and optical properties of the fibers differ among the materials.
Of particular importance is the optical properties of the infrared fibers with respect to the losses therein. Most of the above listed materials have extremely low theoretical losses but the losses actually achieved have generally been much higher than theoretical. In the extruded fibers, scattering due to surface conditions, grain structure and multiphonon absorption appear to be responsible for most of the losses. It is therefore possible to improve the actual losses to approach the theoretical losses by minimizing the scattering loss through improvement of surface quality, reduction of the crystalline grain size, and use of purer starting materials.
The state of the prior art in infrared optical fibers is believed to be defined by the following references:
1. D. Chen, R. SKogman, G. E. Bernal and C. Butter, "Fabrication of Silver Halide Fibers by Extrusion", Fiber Optics: Advances in Research and Development, edited by B. Bendow and S. S. Mitra, Plenum, New York, 1977.
2. D. A. Pinnow, A. L. Gentile, A. G. Stardlee, A. J. Timper, and L. M. Holbrock, "Polycrystalline Fiber Optical Waveguides for Infrared Transmission", Applied Physics Letters, Vol. 33, pp 28-29 (1978).
3. J. A. Garfunkel, R. A. Skogman, and R. A. Walterson, "Infrared Transmitting Fibers of Polycrystalline Silver Halides", 1979 IEEE/OSA Conference on Laser Engineering and Applied Digest of Technology, Paper 8.1.
4. J. A. Harrington, M. Braunstein, B. Bobbs and R. Braunstein, "Scattering Losses in Single and Polycrystalline Infrared Material for Infrared Fiber Applications", Adv. in Ceramics, Vol. 2, pp 94-103 (1981).
5. Sakuragi, S., Saito, M., Kubo, Y., Imagawa, K., Kotani, H., Morikawa, T., Shinada, J., "KRS-5 Optical Fibers Capable of Transmitting High-power CO.sub.2 Laser Beam", Opt. Lett., Vol. 6 (December 1981).
6. Sakuragi, S. "Polycrystalline KRS-5 Infrared Fibers for Power Transmission". Research Report, Agency of Industrial Science and Technology, Ministry of Int. Trade and Industry, Japan, 320-02, 1981.
7. Sakuragi, S., Imagawa, K., Saito, M., Kotani, H., Morikawa, T., Shimada, J., "Infrared Transmission Capabilities of Thallium Halide and Silver Halide Optical Fibers", Adv. in Ceramics, Vol. 2, pp 84-93 (1981).
8. J. A. Harrington, "Crystalline Infrared Fibers".
9. Bendayan, et al, U.S. Pat. No. 4,302,073.
10. Anderson, et al, U.S. Pat. No. 4,253,731.
Chen, in reference 1 above, reports fabricating silver halide (AgCl and AgBr) fibers which were extruded in diameters of 3 to 18 mil at the rate of 0.2 to 25 inches per minute at temperatures from 20.degree. to 300.degree. C. To obtain fine grain size, low extrusion rates and low temperatures were used. Single crystals were used for extrusion. An absorption coefficient of 5.times.10.sup.-3 cm.sup.-1 was measured. Garfunkel (ref. 3) reported that fibers extruded from AgCl and AgBr could be obtained having a fine grained structure initially, but that grain growth occurred rapidly after extrusion. Garfunkel also extruded potassium chloride (KCl) fibers which were hygroscopic and were found to be quite brittle at room temperature. No physical or optical characteristics were reported. The Pinnow paper (ref. 2) discusses extruded fibers of thallium bromide (TlBr) and thallium bromo-iodide (TlBrI, known commerically as KRS-5) which have a polycrystalline form. These were prepared from diameters in the range of 75 to 500 microns and were extruded at temperatures in the range of 200.degree. to 350.degree. C. at rates of several centimeters per minute. The crystaline fibers were inserted into a loose-filling polymer cladding to provide optical confinement and mechanical protection.
Harrington (ref. 4) studied the scattering losses in single crystal and polycrystalline KCl and KRS-5. At IR wavelengths, the scattering and multiphonon absorption mechanisms were identified as the limiting loss processes. The polycrystalline materials were found to scatter more strongly than the single crystal materials. In reference 8, Harrington reported on efforts to improve losses in IR transmissive crystalline materials including KRS-5, TlBr, AgCl, KBr, and KCl. He notes that the silver and thallium halides have high refraction indices which can present problems in finding suitable cladding for single mode operation, while the alkali halides have reasonable refractive indices but are hygroscopic and have high melting points. The measured losses in IR fibers are very much higher than intrinsic values. A table of losses is provided for 10.6 microns (the wavelength of a CO.sub.2 laser energy) comparing intrinsic values, bulk material values and fiber material values of the absorption coefficient and the attenuation per meter, as follows:
______________________________________ EXPERIMEN- INTRINSIC TAL BULK FIBER ______________________________________ KRS-5 1 .times. 10.sup.-6 cm.sup.-1 7 .times. 10.sup.-4 cm.sup.-1 9 .times. 10.sup.-4 cm.sup.-1 4.4 .times. 10.sup.-4 dB/m 0.3 dB/m 0.4 dB/m TlBr 1 .times. 10.sup.-6 cm.sup.-1 1 .times. 10.sup.-3 cm.sup.-1 1 .times. 10.sup.-3 cm.sup.-1 4.4 .times. 10.sup.-4 dB/m 0.43 dB/m 0.43 dB/m AgCl 5 .times. 10.sup.-5 cm.sup.-1 5 .times. 10.sup.-3 cm.sup.-1 9 .times. 10.sup.-3 cm.sup.-1 .022 dB/m 2.18 dB/m 4.0 dB/m KBr 1 .times. 10.sup.-6 cm.sup.-1 1 .times. 10.sup.-5 cm.sup.-1 -- 4.4 .times. 10.sup.-6 dB/m KCl 8 .times. 10.sup.-5 cm.sup.-1 8 .times. 10.sup.-5 cm.sup.-1 1 .times. 10.sup.-2 cm.sup.-1 .035 dB/m 0.034 dB/m 4.2 dB/m ______________________________________
The thallium halides have produced fibers with losses close to the bulk values, but poor results have been obtained for KCl fibers. Power densities in a one meter KRS-5 fiber from 2.5 kW/cm.sup.2 to 6.1 KW/cm.sup.2 were reported. Harrington found that extrusion of KCl produced a poor surface quality resulting from friction between the KCl and the extrusion die and therefore abandoned attempts to extrude this material.
Sakuragi and others have experimented with IR fibers for conducting the output of CO.sub.2 lasers (10.6 .mu.m energy) at high power level. As reported in references 5-7, these workers concluded that a mixed halide fiber such as KRS-5 is superior to the pure halides such as AgCl. An extinction coefficient less than 10.sup.-2 cm.sup.-1 could not be obtained for AgCl. They concluded that KRS-5 was a superior material, mechanically, optically and chemically. To reduce losses it is required to minimize anion impurities such as SO.sub.4.sup.2-, NO.sub.3.sup.- and HCO.sub.3.sup.-. Also, a good surface finish is necesary to minimize inclusions, cracks and scratches which increase scattering losses and decrease the power damage threshold. For example, reference 5 reported that extrusion of KRS-5 through a diamond wire die resulted in 1-2 micron deep scratches on the fiber surface from microscopic dust in the die.
These papers report transmission losses of 0.4 to 0.6 dB/m and extinction coefficients of 1.5.times.10.sup.-3 cm.sup.-1 to 9.times.10.sup.-4 cm.sup.-1 although bulk KRS-5 was measured at 4.times.10.sup.-4 cm.sup.-1, somewhat lower than the value reported by Harrington. Although some of the increase in loss of the extruded fibers is due to scattering loss at the polycrystalline boundaries, it is predicted that the fiber loss can approach the bulk value by reduction of impurities, defects and grain size.
The maximum power density reported is 36 kW/cm.sup.2. Optical degradation can occur from mechanical deformation of the fibers. A minimum ending radius of 12 cm was noted for a 1 mm diameter KRS-5 fiber. Sakuragi describes a surgical probe using the 1 mm KRS-5 fiber. The fiber was covered by a loose-fitting polymer tube for protection.
Commercial KRS-5 fiber products have required bulky protective coatings to protect the fiber, as well as to protect the environment from the toxic KRS-5. This packaging drastically reduces the utility of the fiber by restricting its use to external or open body cavity procedures only. This is due to the inability of such large, flexible devices to be used in conjunction with an endoscope. Horiba, Inc. (Japan) formerly offered a KRS-5 fiber having an outer jacket of 10 mm diameter protecting a 1 mm fiber optic.
As may be understood from the above references, IR fibres have various critical problems that must be addressed in producing practical devices. The outer surface of the fiber must be highly finished with a minimum of cracks or scratches. Both the alkali halides and thallium halides are hygroscopic. Thus, the fibers require some coating for mechanical protection of the outer surface. Also, a restraint on the minimum bending radius must be provided. To minimize the losses in the fibers, the extrusion process must be such that a very fine grain crystalline structure is realized.
The patent to Bendayan, et al (ref. 9) notes that attempts have been made to extrude a tight fitting plastic covering onto an IR fiber such as taught by Hawkins in U.S. Pat. No. 3,742,107. However, the lateral pressure on the fiber results in microfractures of the fiber surface increasing transmission loss. Bendayan, et al teach the extrusion of a plastic covering over an optical fiber with a clearance of 1 to 10 microns therebetween eliminating the lateral pressure.
Reference 10, the Anderson, et al patent, describes a method of extruding a silver bromide IR fiber core having a silver chloride cladding. A coaxial billet is extruded through a diamond die 6 to 18 ml in diameter. The resulting boundary between fiber core and cladding is rough and poorly defined. See FIG. 4 of Anderson.
As referred to herein "clad" or "cladding" means a coaxial crystalline or polycrystalline layer surrounding a fiber core.
As referred to herein "window" shall mean a window or a lens in an infrared optical cable.